@article{yoo_aspnes_2001, title={Elimination of endpoint-discontinuity artifacts in the analysis of spectra in reciprocal space}, volume={89}, ISSN={["0021-8979"]}, DOI={10.1063/1.1368391}, abstractNote={Reciprocal-space analysis offers several advantages for determining critical point parameters in optical and other spectra, for example the separation of baseline effects, information, and noise in low-, medium-, and high-index Fourier coefficients, respectively. However, endpoint-discontinuity artifacts can obscure much of the information when segments are isolated for analysis. We developed a procedure for eliminating these artifacts and recovering buried information by minimizing in the white-noise region the mean-square deviation between the Fourier coefficients of the data and those of low-order polynomials, then subtracting the resulting coefficients from the data over the entire range. We find that spectral analysis is optimized if no false data are used, i.e., when the number of points transformed equals the number of actual data points in the segment. Using fractional differentiation we develop a simple derivation of the variation of the reciprocal-space coefficients with index n for Lorentzian and Gaussian line shapes in direct space. More generally, we show that the definition of critical point energies in terms of phase coherence of the Fourier coefficients allows these energies to be determined for a broad class of line shapes even if the direct-space line shapes themselves are not known. Limitations for undersampled or highly broadened spectra are discussed, along with extensions to two- or higher-dimensional arrays of data.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Yoo, SD and Aspnes, DE}, year={2001}, month={Jun}, pages={8183–8192} }
 @article{koo_kim_lee_oh_kim_yoo_aspnes_jonker_2000, title={Dielectric function of epitaxial ZnSe films}, volume={77}, ISSN={["0003-6951"]}, DOI={10.1063/1.1328098}, abstractNote={We examine various ZnSe spectra to obtain that which best represents the dielectric response ε of ZnSe. The measured evolution of pseudodielectric function 〈ε〉 data with chemical etching shows that the natural overlayer on ZnSe can be modeled accurately only if we assume that it contains amorphous Se. Hence previous assumptions made in correcting 〈ε〉 mathematically are not correct, and data obtained on stripped samples yield the best representation of ε.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Koo, MS and Kim, TJ and Lee, MS and Oh, MS and Kim, YD and Yoo, SD and Aspnes, DE and Jonker, BT}, year={2000}, month={Nov}, pages={3364–3366} }
 @article{hyun_barletta_koh_yoo_oh_aspnes_cuomo_2000, title={Effect of Ar+ ion beam in the process of plasma surface modification of PET films}, volume={77}, ISSN={["0021-8995"]}, DOI={10.1002/1097-4628(20000822)77:8<1679::AID-APP4>3.0.CO;2-F}, abstractNote={In general, plasma modified polymer surfaces tend to show short aging time and rapid hydrophobic recovery after treatment. To prevent reorientation from the surface to the bulk, appropriate crosslinking is necessary among the polymer chains. In this work, an Ar+ ion beam was used to provide crosslinking to the surface. Crosslinking was shown by spectroscopic ellipsometry, AFM, and FTIR. Contact angle measurements were performed to see the aging of the modified surfaces. The surface modified with Ar+ ion beam followed by RF plasma treatment exhibited reduced chain mobility and a highly stable hydrophilic surface. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1679–1683, 2000}, number={8}, journal={JOURNAL OF APPLIED POLYMER SCIENCE}, author={Hyun, J and Barletta, P and Koh, K and Yoo, S and Oh, J and Aspnes, DE and Cuomo, JJ}, year={2000}, month={Aug}, pages={1679–1683} }
 @article{yoo_aspnes_lastras-martinez_ruf_konuma_cardona_2000, title={High-resolution spectroscopy with reciprocal-space analysis: Application to isotopically pure Si}, volume={220}, ISSN={["1521-3951"]}, DOI={10.1002/1521-3951(200007)220:1<117::aid-pssb117>3.0.co;2-4}, abstractNote={We discuss a new Fourier-transform approach that has recently been developed to optimize the determination of critical point parameters in optical spectra. In this approach, segments of direct (energy or frequency) space spectra are Fourier transformed into reciprocal (Fourier-inverse) space, and the endpoint-discontinuity artifacts that result are eliminated by subtracting corresponding coefficients of low-order Legendre polynomials determined by least-squares fitting these coefficients to the transformed data in the white-noise region. We apply this approach to determine the extremely small effect of isotopic mass on the energy of the E1 critical point of crystalline Si from low-temperature spectroscopic ellipsometric data.}, number={1}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Yoo, SD and Aspnes, DE and Lastras-Martinez, LF and Ruf, T and Konuma, M and Cardona, M}, year={2000}, month={Jul}, pages={117–125} }
 @article{ebert_bell_yoo_flock_aspnes_2000, title={In situ monitoring of MOVPE growth by combined spectroscopic ellipsometry and reflectance-difference spectroscopy}, volume={364}, ISSN={["1879-2731"]}, DOI={10.1016/S0040-6090(99)00920-7}, abstractNote={Comprehensive characterization of epitaxial growth by metal organic vapor phase epitaxy (MOVPE) requires a combination of thin-film, near-surface-, and surface-sensitive techniques to determine layer thicknesses and compositions, composition of the most recently deposited material, and surface chemistry, respectively. These data can be obtained non-destructively by spectroscopic ellipsometry (SE) and reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). Here we describe the first unified optical system, basically a rotating-polarizer ellipsometer (RPE) integrated into a modified commercial rotating-sample MOVPE reactor, that performs both SE and RDS simultaneously with a single optical path. Data are obtained in parallel from 240 to 840 nm with a high-speed 16-bit photodiode array (PDA) at a repetition rate greater than 2 Hz and a precision of ±0.0001. We provide examples of its use, and show in particular that GaP intermixes with Si during the initial stages of heteroepitaxy. Capabilities of the presented configuration and its potential for future investigations are discussed.}, number={1-2}, journal={THIN SOLID FILMS}, author={Ebert, M and Bell, KA and Yoo, SD and Flock, K and Aspnes, DE}, year={2000}, month={Mar}, pages={22–27} }
 @article{edwards_bremser_batchelor_buyanova_madsen_yoo_welhkamp_wilmers_cobet_esser_et al._2000, title={Optical characterization of wide bandgap semiconductors}, volume={364}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(99)00903-7}, abstractNote={Our work primarily concerns the characterization of wide-gap III-V nitride semiconductors, nondestructively and at variable temperature, with spectroscopic ellipsometry (SE) and reflectometry in the spectral range from 1.5 to 6 eV. In the case of GaN, there are three main concerns associated with such data: (a) the quantification of the dispersion of the index of refraction with energy, (b) the removal of surface overlayers in real-time, and (c) the determination of the variation of valence bands with biaxial stress and the quantification of residual stress in thin films. The SE and reflectance capabilities provide (1) broadband spectra from 1.5 to 6 eV, which yield information about (a) below the bandgap and (b) above it, and (2) high resolution spectra (less than 1 meV at 3.4 eV) in the vicinity of the gap (3.3-3.6 eV), which enables (c). Here we will discuss issues concerning the relation of (c) to GaN material and growth parameters, though similar data for other wide bandgap materials will be discussed where relevant. Specifically, optimal heterostructure design for potential valence band engineering applications will be discussed in the context of trends in residual stress as a function of film thickness, growth temperature and substrate orientation for GaN/AlN/6H-SiC heterostructures. Standard heterostructures are mostly compressive for samples less than about 0.7 µm thick, are tensile up to about 2 µm and then abruptly become less tensile with stress values near 1 kbar thereafter. Additionally, these trends can be circumvented for moderately thick (approximately 2 µm) GaN layers (normally>2 kbar, tensile) by the introduction of a `buried interface' approach, namely, a strain mediating layer (SML) above the standard high-temperature AlN buffer layer designed to yield a range of compressive stresses from 0 to 2 kbar. The strain characteristics but also the growth rates of subsequently deposited nitride layers can be modulated by changing the growth parameters of the SML. This is achieved by in situ techniques during crystal growth without degrading the optical and structural properties of the deposited layer, as confirmed by XRD, SEM, PL, and AFM data taken on the overlying GaN layers. These results are interpreted in terms of coefficient of thermal expansion data for the layers and data concerning the planarization of GaN layers and growth behavior in non-(0001) directions.}, number={1-2}, journal={THIN SOLID FILMS}, author={Edwards, NV and Bremser, MD and Batchelor, AD and Buyanova, IA and Madsen, LD and Yoo, SD and Welhkamp, T and Wilmers, K and Cobet, C and Esser, N and et al.}, year={2000}, month={Mar}, pages={98–106} }
 @article{choi_kim_yoo_aspnes_woo_kim_2000, title={Optical properties of AlxGa1-xP (0 <= x <= 0.52) alloys}, volume={87}, ISSN={["0021-8979"]}, DOI={10.1063/1.372011}, abstractNote={We report optical properties of AlxGa1−xP (0⩽x⩽0.52) alloys grown by gas source molecular-beam epitaxy on S-doped GaP(001) substrates. Room-temperature pseudodielectric function spectra from 1.5 to 6.0 eV were obtained by spectroscopic ellipsometry. By applying the parabolic-band critical point model to numerically calculated second energy derivatives of these spectra, we obtained accurate room-temperature values of the E1, E0′, E2, and E2′ critical point energies and their dependence on composition x.}, number={3}, journal={JOURNAL OF APPLIED PHYSICS}, author={Choi, SG and Kim, YD and Yoo, SD and Aspnes, DE and Woo, DH and Kim, SH}, year={2000}, month={Feb}, pages={1287–1290} }
 @article{bell_ebert_yoo_flock_aspnes_2000, title={Real-time optical characterization of heteroepitaxy by organometallic chemical vapor deposition}, volume={18}, ISSN={["0734-2101"]}, DOI={10.1116/1.582323}, abstractNote={Heteroepitaxy of GaP on Si(100) and GaAs(100) is investigated under organometallic chemical vapor deposition conditions using combined spectroscopic ellipsometry (SE) and non-normal-incidence reflectance-difference (-anisotropy) spectroscopy. Real-time monitoring greatly assists in identifying optimum starting surfaces for heteroepitaxy since prolonged exposure to PH3 results in roughening of Si(100) and GaAs(100) surfaces, in agreement with previous work. Real-time SE data of GaP on Si indicate that under our conditions GaP and Si interpenetrate as optically identifiable materials over the first 75 Å, suggesting that either trimethylgallium or a reaction by-product can act as a catalyst for the formation of Si{111} facets.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Bell, KA and Ebert, M and Yoo, SD and Flock, K and Aspnes, DE}, year={2000}, pages={1184–1189} }
 @article{bell_ebert_yoo_flock_aspnes_2000, title={Real-time optical techniques and QMS to characterize growth in a modified commercial OMVPE reactor}, volume={29}, ISSN={["0361-5235"]}, DOI={10.1007/s11664-000-0104-6}, number={1}, journal={JOURNAL OF ELECTRONIC MATERIALS}, author={Bell, KA and Ebert, M and Yoo, SD and Flock, K and Aspnes, DE}, year={2000}, month={Jan}, pages={106–111} }
 @article{kim_kim_yoo_aspnes_kossut_1999, title={Dielectric function of Cd0.57Mg0.43Te alloy film studied by ellipsometry}, volume={34}, number={1999 June}, journal={Journal of the Korean Physical Society}, author={Kim, T. J. and Kim, Y. D. and Yoo, S. D. and Aspnes, D. E. and Kossut, J.}, year={1999}, pages={S496–498} }
 @article{aspnes_yoo_1999, title={High-resolution spectroscopy with reciprocal-space analysis}, volume={215}, ISSN={["0370-1972"]}, DOI={10.1002/(sici)1521-3951(199909)215:1<715::aid-pssb715>3.0.co;2-g}, abstractNote={Optical spectra can be analyzed more accurately for critical point energies and other parameters in reciprocal space than in direct space because baseline effects, information, and noise are effectively isolated in the low-, medium-, and high-index Fourier coefficients, respectively. However, the requirement that the spectral segment being analyzed is periodic usually gives rise to large contributions to the coefficients from the value and slope discontinuities at the wrapped ends of the segment. Previously, the effect of these discontinuities was reduced with false data in direct space. Here, we show that these artifacts can be removed more effectively in reciprocal space, and that the process is optimized when the number of data points in the segment equals the number of points in the transformation, i.e., when no false data are required. Our approach reduces reciprocal-space analysis of optical spectra to a routine process, as demonstrated by application to crystalline Si.}, number={1}, journal={PHYSICA STATUS SOLIDI B-BASIC RESEARCH}, author={Aspnes, DE and Yoo, SD}, year={1999}, month={Sep}, pages={715–723} }
 @article{yoo_edwards_aspnes_1998, title={Analysis of optical spectra by Fourier methods}, volume={313}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(97)00801-8}, abstractNote={Results of a systematic investigation of reciprocal-space analysis, applied to optical spectroscopy, are reported. Filtering procedures for removing baseline and noise artifacts are more effective in reciprocal- than in real-space. Also, correlations among parameters are reduced and the functional dependence of real-space lineshapes need be known a priori only in very general terms. We apply reciprocal-space analysis to achieve accurate values of critical point energies for the E1 and E1+Δ1 transitions of GaAs from ellipsometric spectra and to locate critical point energies in low-temperature reflectance data of GaN. We show that data may easily be corrected for systematic artifacts such as the monochromator slit width, an optimum slit width can be defined, and the degree of improvement needed to achieve a particular level of performance can be predicted.}, number={1998 Feb.}, journal={THIN SOLID FILMS}, author={Yoo, SD and Edwards, NV and Aspnes, DE}, year={1998}, month={Feb}, pages={143–148} }
 @article{woo_han_choi_lee_kim_lee_kim_kang_choi_kim_et al._1998, title={Optical characterization of GaAs/AlAs short period superlattices}, volume={43-4}, ISSN={["0167-9317"]}, DOI={10.1016/S0167-9317(98)00173-7}, abstractNote={We present optical studies of a series of GaAs/AlAs short period superlattices grown by molecular beam epitaxy. The structural properties were examined by X-ray diffraction measurements. The quantum confinement of the electronic and the vibronic states were observed in the low temperature photoluminescence and Raman scattering experiments, respectively. Spectroscopic ellipsometric (SE) measurements were also performed. We found a new structure at the lower E2 peak, which is the best resolution of the E2 structure in these SLs so far obtained by SE.}, number={1998 Aug.}, journal={MICROELECTRONIC ENGINEERING}, author={Woo, DH and Han, IK and Choi, WJ and Lee, S and Kim, HJ and Lee, JI and Kim, SH and Kang, KN and Choi, SG and Kim, YD and et al.}, year={1998}, month={Aug}, pages={265–270} }
 @article{edwards_yoo_bremser_horton_perkins_weeks_liu_stall_kuech_davis_et al._1998, title={Spectroscopic ellipsometry and low-temperature reflectance: complementary analysis of GaN thin films}, volume={313}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(97)00815-8}, abstractNote={Abstract We report spectroscopic ellipsometry (SE) and low-temperature reflectance data on epitaxial GaN thin-film samples covering the widest range of tensile and compressive stress (−3.8 to 3.5 kbar) thus far. SE allows us to assess the preparation of smooth and abrupt GaN surfaces by chemical treatments in real time, and, coupled with the reflectance data, the E d n /d E contribution to dispersion, which is important for laser action. The reflectance data explicitly show the non-linear behavior of the B-A and C-A splittings vs. the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal space analysis, allowing us to obtain band parameters such as Δ SO =17.0±1 meV and Δ CF =9.8±1 meV with increased confidence.}, number={1998 Feb.}, journal={THIN SOLID FILMS}, author={Edwards, NV and Yoo, SD and Bremser, MD and Horton, MN and Perkins, NR and Weeks, TW and Liu, H and Stall, RA and Kuech, TF and Davis, RF and et al.}, year={1998}, month={Feb}, pages={187–192} }
 @article{edwards_bremser_davis_batchelor_yoo_karan_aspnes_1998, title={Trends in residual stress for GaN/AlN/6H-SiC heterostructures}, volume={73}, ISSN={["1077-3118"]}, DOI={10.1063/1.122597}, abstractNote={We discuss trends in residual stress as a function of film thickness, growth temperature, and substrate orientation for GaN/AlN/6H–SiC heterostructures. Films are mostly compressive for samples less than about 0.7 μm thick, are tensile up to about 2 μm, then abruptly become less tensile with stress values near 1 kbar thereafter. We interpret this as a successive relief of lattice mismatch and thermal stresses culminating in a catastrophic relief by unknown mechanisms at moderate thicknesses. These data indicate that relaxation processes in these heterostructures are not as well understood as previously supposed.}, number={19}, journal={APPLIED PHYSICS LETTERS}, author={Edwards, NV and Bremser, MD and Davis, RF and Batchelor, AD and Yoo, SD and Karan, CF and Aspnes, DE}, year={1998}, month={Nov}, pages={2808–2810} }
 @article{kim_ko_choi_yoo_aspnes_jonker_1997, title={Above bandgap dielectric function of epitaxial ZnSe layers}, volume={31}, number={4}, journal={Journal of the Korean Physical Society}, author={Kim, Y. D. and Ko, Y. D. and Choi, S. G. and Yoo, S. D. and Aspnes, D. E. and Jonker, B. T.}, year={1997}, pages={L553–555} }
 @article{choi_kim_yoo_duk_miotkowski_i._ramdas_1997, title={Ellipsometric studies of Cd1-xMgxTe (0<=x<=0.5) alloys}, volume={71}, ISSN={["1077-3118"]}, DOI={10.1063/1.119511}, abstractNote={The determination of the above band gap optical properties of zincblende Cd1−xMgxTe (0⩽x⩽0.5) ternary alloys are reported on. Using the parabolic-band critical point model, room-temperature critical point energies of the E0, E0+Δ0, E1, E1+Δ1, E2, and E0′ interband transitions from numerically calculated second energy derivatives of ellipsometric spectra were obtained. The presence of two distinct structures in the E2 feature for x>0 was also observed.}, number={2}, journal={APPLIED PHYSICS LETTERS}, author={Choi, S. G. and Kim, Y. D. and Yoo, Sang and Duk, Aspnes and Miotkowski, David E. and I. and Ramdas, A. K.}, year={1997}, month={Jul}, pages={249–251} }
 @article{edwards_yoo_bremser_zheleva_horton_perkins_weeks_liu_stall_kuech_et al._1997, title={Spectral analysis of above-, below-, and near-bandedge phenomena in GaN thin films}, volume={50}, ISSN={["0921-5107"]}, DOI={10.1016/s0921-5107(97)00151-7}, abstractNote={We report spectroscopic ellipsometry (SE) and low-temperature reflectance data on epitaxial GaN thin film samples covering the widest range of tensile and compressive stress (−3.8–3.5 kbar) thus far. SE allows us to assess the preparation of smooth and abrupt GaN surfaces by chemical treatments in real time, and, coupled with the reflectance data, the E dn/dE contribution to dispersion, which is important for laser action. The reflectance data explicitly show the nonlinear behavior of the B–A and C–A splittings versus the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal space analysis, allowing us to obtain band parameters such as ΔSO=17.0±1 meV and ΔCF=9.8±1 meV with increased confidence.}, number={1-3}, journal={MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY}, author={Edwards, NV and Yoo, SD and Bremser, MD and Zheleva, T and Horton, MN and Perkins, NR and Weeks, TW and Liu, H and Stall, RA and Kuech, TF and et al.}, year={1997}, month={Dec}, pages={134–141} }
 @article{kim_choi_klein_yoo_aspnes_xin_furdyna_1997, title={Spectroscopic ellipsometric characterization of undoped ZnTe films grown on GaAs}, volume={70}, ISSN={["0003-6951"]}, DOI={10.1063/1.118289}, abstractNote={We report highly accurate dielectric function data for ZnTe. These data were made possible by the high quality of the heteroepitaxial material and the development of a chemical etching procedure for producing abrupt surfaces on ZnTe; they provided the first observation of the E0+Δ0 structure in ZnTe by spectroscopic ellipsometry and evidence for several contributions to the E2 structure. Accurate critical point energies were obtained by Fourier analysis.}, number={5}, journal={APPLIED PHYSICS LETTERS}, author={Kim, YD and Choi, SG and Klein, MV and Yoo, SD and Aspnes, DE and Xin, SH and Furdyna, JK}, year={1997}, month={Feb}, pages={610–612} }
 @article{choi_kim_klein_yoo_aspnes_xin_furdyna_1997, title={Spectroscopic ellipsometric study of Zn(1-x)Mn(x)Te films grown on GaAs}, volume={31}, number={1}, journal={Journal of the Korean Physical Society}, author={Choi, S. G. and Kim, Y. D. and Klein, M. V. and Yoo, S. D. and Aspnes, David E. and Xin, S. H. and Furdyna, J. K.}, year={1997}, pages={202–205} }
 @article{choi_kim_yoo_aspnes_rhee_woo_woo_kim_kang_1997, title={Spectroscopic ellipsometry study of GaAs/AlAs superlattices and Al(0.5)Ga(0.5)As alloy}, volume={30}, number={suppl.}, journal={Journal of the Korean Physical Society}, author={Choi, S. G. and Kim, Y. D. and Yoo, S. D. and Aspnes, David E. and Rhee, S. J. and Woo, J. C. and Woo, D. H. and Kim, S. H. and Kang, K. N.}, year={1997}, pages={108–112} }
 @article{edwards_yoo_bremser_weeks_nam_davis_liu_stall_horton_perkins_et al._1997, title={Variation of GaN valence bands with biaxial stress and quantification of residual stress}, volume={70}, ISSN={["0003-6951"]}, DOI={10.1063/1.119089}, abstractNote={Low-temperature reflectance data on epitaxial GaN thin-film samples covering the widest range of tensile and compressive stress (−3.8–3.5 kbar) thus far explicitly show the nonlinear behavior of the B–A and C–A splittings versus the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal-space analysis, allowing us to obtain band parameters such as ΔSO=17.0±1meV with increased confidence.}, number={15}, journal={APPLIED PHYSICS LETTERS}, author={Edwards, NV and Yoo, SD and Bremser, MD and Weeks, TW and Nam, OH and Davis, RF and Liu, H and Stall, RA and Horton, MN and Perkins, NR and et al.}, year={1997}, month={Apr}, pages={2001–2003} }
 @inproceedings{edwards_yoo_bremser_horton_perkins_weeks_liu_stall_kuech_davis_et al._1997, title={Variation of GaN valence bands with biaxial stress: quantification of residual stress and impact on fundamental band parameters}, DOI={10.1557/proc-449-781}, abstractNote={ABSTRACT}, booktitle={III-V nitrides: symposium held December 2-6, 1996, Boston, Massachusetts, U.S.A.  (Materials Research Society symposia proceedings ; v. 449)}, publisher={Pittsburgh, Pa.: Materials Research Society}, author={Edwards, N. V. and Yoo, S. D. and Bremser, M. D. and Horton, M. N. and Perkins, N. R. and Weeks, T. W. and Liu, H. and Stall, R. A. and Kuech, T. F. and Davis, R. F. and et al.}, year={1997}, pages={781–786} }